Forgot Password
Pentax Camera Forums Home
 

Reply
Show Printable Version Search this Thread
03-12-2009, 05:01 AM   #1
Forum Member




Join Date: Mar 2009
Location: LA
Posts: 80
How to check Back Focus/Front Focus

Hi guys,

I am new to DSLR's!
I am about to get my first DSLR k200d. After reading tons of reviews and faq's about this camera, I would like to know how to check BF/FF? or .. hmmm.... let's say in case I will have this problem, how can I fix it? I am thinking about getting the kit version.

thanks.

03-12-2009, 07:49 AM   #2
Behind-Camera
Guest




QuoteOriginally posted by grishazzz Quote
Hi guys,

I am new to DSLR's!
I am about to get my first DSLR k200d. After reading tons of reviews and faq's about this camera, I would like to know how to check BF/FF? or .. hmmm.... let's say in case I will have this problem, how can I fix it? I am thinking about getting the kit version.

thanks.
Hello. Well http://focustestchart.com/focus21.pdf should help you.
03-12-2009, 08:49 AM   #3
Forum Member




Join Date: Mar 2009
Location: LA
Posts: 80
Original Poster
thank you!!!
very good FAQ!
03-12-2009, 09:07 AM   #4
Veteran Member
heliphoto's Avatar

Join Date: May 2008
Location: Region 5
Photos: Gallery
Posts: 2,540
Personally, I don't like that chart all that much, though it can be a bit helpfull. I've described one technique I use in this thread, you could use the chart, or my angled dvd boxes, or just any real life scene with a definite focus point which you're sure the camera has locked on to, to assess whether your camera has a marked FF or BF tendency. If it does, you'll have to send it to Pentax for adjustment (unlike the K20D which has a menu option for user correction of mis-focus - this is the #1 reason I'm glad I upgraded to the K20D from the K200D).

03-12-2009, 09:42 AM   #5
Behind-Camera
Guest




When I had Pentax DL after changing the lens I quickly entered the Service Menu and did an AF correction, of course it was slow but it was way better than shooting with BF or FF
03-12-2009, 09:52 AM   #6
Veteran Member
heliphoto's Avatar

Join Date: May 2008
Location: Region 5
Photos: Gallery
Posts: 2,540
QuoteOriginally posted by Behind-Camera Quote
When I had Pentax DL after changing the lens I quickly entered the Service Menu and did an AF correction, of course it was slow but it was way better than shooting with BF or FF
I didn't know that you could do this on that camera (*ist DL right?) I totally agree with you that we need to be able to make focus adjustments on any Pentax camera, not just the K20D. Hopefully, someone out there is working on a hack for the K200D firmware - like what has been created for the K10D. User focus adjustment just makes sense.
03-12-2009, 09:59 AM   #7
Behind-Camera
Guest




QuoteOriginally posted by heliphoto Quote
I didn't know that you could do this on that camera (*ist DL right?) I totally agree with you that we need to be able to make focus adjustments on any Pentax camera, not just the K20D. Hopefully, someone out there is working on a hack for the K200D firmware - like what has been created for the K10D. User focus adjustment just makes sense.
Yep, on ist* DL there was a combination of keys you had to push when you turn on your camera, then you can enable the debug mode and enter service menu. The AF correction you made affected all the lenses, so you had to do this every time you change the lens.
03-12-2009, 10:32 AM   #8
Forum Member




Join Date: Mar 2009
Location: LA
Posts: 80
Original Poster
thank you all!!!

ok, right after I will get my camera, I will use dvd boxes as a target, exactly in a same position/combination like the one shown here. and then I'll see if there is bb/ff.

Please correct me if I am wrong:

let's say: I will have two lenses, kit and tele like da 55-300.
now, if the kit lens works fine, but the telephoto 55-300 has some problems with focusing, it means - my copy of this lens is defective? Or there is a problem in camera?
I am sorry for my stupid questions, I've never had DSLR before only P$S (canon a620/ canon g7), so I am trying to understand how to check and avoid focusing issues with new dslr camera or in case it's defective send it back for exchange/repair.

btw:
I have not ordered camera yet, still thinking about k200d, or k20d which has debug mode option for fixing BB/FF issues.

03-12-2009, 11:14 AM   #9
Veteran Member
heliphoto's Avatar

Join Date: May 2008
Location: Region 5
Photos: Gallery
Posts: 2,540
If you can afford the K20D, it is a superior camera and has a built in option for fixing BF/FF issues (unless the problem is very bad). I had the K200D first and it's great too, but I love the features of the K20D, and if your focus needs adjustment (which it might not), the K20D allows you to do that.

I'd recommend that right after you get your camera, you go outside and take pictures . Don't worry about little technical details like BF/FF for at least a week . I'd recommend that if you do find that your camera has a focusing issue (that you can't fix yourself) that you send it to Pentax for warranty work rather than sending it back to the store to get another. What if the store sends you another one which also has focusing issues? If you send it to Pentax, they will most likely fix the problem and return your calibrated camera, and then you'll know you've got a properly calibrated one (probably ).

Unless BF/FF is very very bad, it will be hard to notice in normal shooting, so enjoy your new camera... It's only when using lens and aperture combinations to acheive very narrow Depth of Field that focusing errors are most noticeable. If you find in a few weeks that the camera is back focusing or whatever, then you can try to adjust it (with the K20D) or send it for service.

If you do want to check the ff/bf tendencies, the DVD example I used is one of many tests I do when checking a lens. I will also go outside and shoot things like a fence (stand right next to the fenc, and focus on a fence post about 10 feet away, and see if the railing is more in focusing behind or in front of the post closer to you, or is the railing most in focus right next to the post where you focused)... For all such testing, you'll want to set your lens to give you the shallowest depth of field by setting your widest open aperture setting (lowest number f/stop - like f/3.5 vs. f/8 or f/11). Keep in mind with a zoom lens that the focus might be out for the lens at one length, but right on for a different length, and likewise, a lens might focus wrong on close targets but focus perfectly on something 30' away, and you can't necessarily correct it to work perfectly for both situations. Learn to manually focus your lens when necessary, and you'll never need to blame your camera for messing up your focusing again (it'll be your fault then - but seriously, manually focusing can be VERY helpful, it's a great skill to have).
03-12-2009, 01:27 PM   #10
Veteran Member
krypticide's Avatar

Join Date: Dec 2007
Photos: Gallery
Posts: 1,079
If you are doing shoots for clients, absolutely make sure focus is working properly. I blew a paid shoot due to not checking focus carefully in the LCD, and now I'm super paranoid about focus...
03-17-2009, 07:56 AM   #11
Forum Member




Join Date: Mar 2009
Location: LA
Posts: 80
Original Poster
thanks again for your answers.
03-17-2009, 02:51 PM   #12
Senior Member
rpriedhorsky's Avatar

Join Date: Dec 2008
Location: Minneapolis
Posts: 210
QuoteOriginally posted by heliphoto Quote
Personally, I don't like that chart all that much, though it can be a bit helpfull. I've described one technique I use in this thread, you could use the chart, or my angled dvd boxes, or just any real life scene with a definite focus point which you're sure the camera has locked on to, to assess whether your camera has a marked FF or BF tendency. If it does, you'll have to send it to Pentax for adjustment (unlike the K20D which has a menu option for user correction of mis-focus - this is the #1 reason I'm glad I upgraded to the K20D from the K200D).
I checked my camera using ad-hoc methods (tilted book) and those showed that there was a big front-focus issue. But, testing with a chart showed no problems. Based on this experience, I would suggest using a test chart.

I used a different chart than the focustestchart.com one linked from this forum, which involved cutting out and folding. No link, unfortunately. Note also that the description in that PDF of how DSLR AF sensors work is wrong; he's describing contrast-detect AF, which is how cheap digicams work. I read a great explanation a few weeks back, but again no link, sorry.
03-17-2009, 10:26 PM   #13
Veteran Member
krypticide's Avatar

Join Date: Dec 2007
Photos: Gallery
Posts: 1,079
I recommend Jeffrey Friedl's focus chart over the one that was previously posted. The thin black box on the D70 focus chart may possibly affect the focus if it is not much larger than the AF point.
03-18-2009, 12:26 AM   #14
Veteran Member
heliphoto's Avatar

Join Date: May 2008
Location: Region 5
Photos: Gallery
Posts: 2,540
QuoteOriginally posted by rpriedhorsky Quote
I checked my camera using ad-hoc methods (tilted book) and those showed that there was a big front-focus issue. But, testing with a chart showed no problems. Based on this experience, I would suggest using a test chart.

I used a different chart than the focustestchart.com one linked from this forum, which involved cutting out and folding. No link, unfortunately. Note also that the description in that PDF of how DSLR AF sensors work is wrong; he's describing contrast-detect AF, which is how cheap digicams work. I read a great explanation a few weeks back, but again no link, sorry.
So since you would suggest the chart, I'm assuming your camera exhibits no FF in real life conditions, but it did with the ad-hoc methods? I had the opposite experience with my camera - it focused right on with the chart, but often back focused in real life, so I found a better technique (for me) than the focustestchart.com chart - admittedly there are some new charts coming on the scene which look intriguing but I haven't tried 'em).

Two points on this (but I'm no expert and really don't know what I'm talking about )-
1. I think it's likely much more common to shoot objects which resemble details on a tilted book rather than isolated black lines in a field of white, or black to white contrast changes depending on your chart of choice... for instance, an object on the ground viewed from a standing position, someone's face angled other than straight on, the slope of a mountain, etc... all present an angled surface which the camera has to deal with.

2. In order to eliminate ambiguity for focus testing, I actually advocate focusing on a flat surface parallel with the camera, with other surfaces angled toward and away from the camera or sometimes oriented the same way as the focusing surface, but staggered in front of and behind the focus target surface. I've seen a focus chart for sale that did this - had a flat focus target with an angled scale next to it - IMHO that's a good idea.

What it comes down to is finding a methodology which works for you and makes your focus sharp for real world shooting.
03-18-2009, 11:35 AM   #15
Pentaxian
Marc Sabatella's Avatar

Join Date: Sep 2006
Location: Denver, CO
Photos: Gallery
Posts: 10,686
QuoteOriginally posted by heliphoto Quote
I had the opposite experience with my camera - it focused right on with the chart, but often back focused in real life, so I found a better technique (for me) than the focustestchart.com chart - admittedly there are some new charts coming on the scene which look intriguing but I haven't tried 'em).

Two points on this (but I'm no expert and really don't know what I'm talking about )-
1. I think it's likely much more common to shoot objects which resemble details on a tilted book rather than isolated black lines in a field of white, or black to white contrast changes depending on your chart of choice...
Even though I kind of know better, for the sake of argument I'm taking you at your word that you don't know what you're talking about :-) Actually, my comments are mostly directed at people who *haven't* thought through these issues before:

While it is true that focus charts are by their natural artificial, the advantage they have, in theory, is guaranteeing that the camer is at least *trying* to focus on what you think it is. As has been observed elsewhere, the AF sensors are actually very large - far to large to allow you to focus on a single line of text if there is another line of text above or below it. The camera light legitimately choose any of these lines of text. And then you won't be able to tell if the camera actually *succeeded* in focusing on what it tried to focus on. That's why tests that isolate the target from anything that could possible compete with it are essential.

As long as one understands this, there is nothing wrong with ad hoc tests - you just have to make sure your ad hoc tests presents a similarly unambiguous target. Otherwise, all you are doing is testing the camera's ability to read you mind regarding what to focus on, and I can tell you right now it's going to do no better than random chance on that.

In general, if a camera does well on a focus test but bad in real life, chances are excellent that this is what is happening - the camera has no FF or BF issues but is simply not focusing on the specific target you wanted to focus on, which is often an unavoidable problem (although of course you should at least be careful you are selecting the proper focus point and trying to center it within your target to minimize the likelihood of the amera choosing something else).

But it's also possible that the lens has a FF/BF problem that isn't visible at the close-focus distances that are usually necessary with focus test charts. So an ad hoc test using "real world" focus distances, but that still successfully isolates the focus target from the background, would make sense.
Reply

Bookmarks
  • Submit Thread to Facebook Facebook
  • Submit Thread to Twitter Twitter
  • Submit Thread to Digg Digg
Tags - Make this thread easier to find by adding keywords to it!
camera, check, pentax help, photography
Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
Trying to understand front focus/back focus thing 41ants Troubleshooting and Beginner Help 16 08-31-2010 08:42 AM
front focus, back focus or other? imfinetoday Pentax SLR Lens Discussion 23 07-08-2010 11:49 AM
Front/back focus or focus shift? fulcrumx29 Pentax SLR Lens Discussion 0 08-17-2009 04:56 PM
Front focus/ back focus on Manual lens. Possible? WangJianWei Troubleshooting and Beginner Help 6 04-20-2009 07:50 PM
Front focus/back focus sebberry Troubleshooting and Beginner Help 5 03-23-2009 03:41 PM



All times are GMT -7. The time now is 11:37 AM. | See also: NikonForums.com, CanonForums.com part of our network of photo forums!
  • Red (Default)
  • Green
  • Gray
  • Dark
  • Dark Yellow
  • Dark Blue
  • Old Red
  • Old Green
  • Old Gray
  • Dial-Up Style
Hello! It's great to see you back on the forum! Have you considered joining the community?
register
Creating a FREE ACCOUNT takes under a minute, removes ads, and lets you post! [Dismiss]
Top